The kinetic modeling parts of the model simply assign masses and radii of gyration to the segments defined in the kinematic model. An estimate of the position of the center of mass is required in the segment. This is defined as a point at a given proportion along a line from the distal joint center (normally the origin of the segment) towards the proximal joint center of a "typical" segment. The masses of each segment are calculated as a proportion of the total body mass. The principal axes moments of inertia are calculated from (mass) normalized radii of gyration from these tables too. In general the moment is considered to be zero around the longitudinal axis of most segments. Since experimental data were not available, estimates have been made for the radii of gyration of the pelvis and thorax. You can change these values in the Properties pane of the Process Dynamic Plug-in Gait Model pipeline operation.
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Even though the "untortioned" tibiae are used for the reference frames, a difference in moments will be observed if the trial is processed with a different tibial torsion. When the tibial torsion is applied in the static trial, the ankle joint center is moved backwards, then the "untortioned" tibia is calculated by rotating the tortioned tibia round the Z axis, keeping the ankle joint center in position. Thus, for a given trial, as tibial torsion is increased, and the joint center is rotated backwards around the ankle marker, the ankle flexion moment will generally become more positive.
| SegmentSegment | CoM | Mass | Radius of gyration |
|---|---|---|---|
| Pelvisa | 0.895 | 0.142 | 0.31 |
| Femur | 0.567 | 0.1 | 0.540 |
| Tibia | 0.567 | 0.0465 | 0.528 |
| Foot | 0.5 | 0.0145 | 0.690 |
| Humerus | 0.564 | 0.028 | 0.542 |
| Radius | 0.57 | 0.016 | 0.526 |
| Handb | 00.6205 | 0.006 | 0.440 |
| Thoraxc | 0.63 | 0.355 | 0.31 |
| Headd | see below | 0.081 | 1.116 |
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b) Hand: The length of the hand in this model is defined as the distance from the wrist joint center to the finger tip. An estimate of 0.75 is taken as the proportion of this length to the "Knuckle II" reference point referred to in the Dempster data.
c) Thorax: The thorax length is taken as the distance between an approximation to the C7 vertebra and the L5 vertebra in the Thorax reference frame. C7 is estimated from the C7 marker, and offset by half a marker diameter in the direction of the X axis. L5 is estimated from the L5 provided from the pelvis segment, but localized to the thorax, rather than the pelvis. The positions are calculated for all frames in the trial, and averaged to give the mean length. The Center of mass is deemed to lie at a proportion of 0.63 along this line. The radius of gyration for the thorax is the default user value in the Dynamic Plug-in Gait operation.
d) Head: If you're calculating the Head CoM yourself, note that the Head Origin is shifted from the midpoint between the front markers using a (-0.84, 0, -0.3) mesh offset multiplied by a headscale value (2 * distance between temples). An optional static head rotation offset is also factored in. The center of mass of the head is defined as being 0.52 * the distance from the front to the back of the head along the X axis from the head origin (the midpoint of the front head markers). The length of the head used for the inertial normalization is the distance from this point to the C7 vertebra (the mean position localized to the head segment). The inertia value for the head is applied around all three axes.
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Whole body center of mass
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